The medium posted an ad on Craigslist posing as a 22-year-old woman looking for a rent-free place to ascertain the truthfulness of the practice and was amazed at the response it got.

Within days our reporter was flooded with messages from men offering free rooms.

One said they had a one-bed flat on the side of their house with “full bathroom, bedroom and living room”.

Some claimed they only required the room to be kept clean.

One got in touch to ask: “Do you have a photo?”

A “tall, submissive guy” said he would “not be after any money, but for a woman in the spare room to be bossy with me, telling me what to do round the house”.

One said he did not have a spare room but that our investigator could “sleep in my bed with me”.

Another landlord said he would accept either payment or “sex, kinky sex, maybe three to four times in a week, day or night”.

Andrew Wallis, who is chief executive of anti-slavery charity Unseen, raised concerns about the vulnerability of tenants entering into these kind of arrangements.

“I think landlords placing these ads are treading as close to the line as they can in terms of breaking the law,” he said.

“It’s as close as you can get without stepping over the line.

“It’s playing on vulnerable people and putting them in a situation which makes them further vulnerable.”

Exploitation was his “primary concern”.

“We know there is a thin line between what is exploitative as defined under the Modern Slavery Act,” he said.

“We are calling for a change in the law.”

He urged Craigslist and other organisations to have a “good hard look” at the problem.

“When your platform is used to exploit people you have a moral responsibility to police it better,” Mr. Wallis said.

However, Welsh Government has condemned the practice, saying it is ‘abhorrent practice’ that took advantage of ‘poverty and social inequality’.

A spokesman said: “We are already working to address the underlying difficulties faced by people seeking to secure good-quality, secure accommodation through the provision of affordable housing, mitigating the economic pressures from zero hours contracts and benefit changes, our anti-slavery policies and initiatives and our support services for vulnerable people with multiple needs.”

Craigslist did not respond to multiple requests for a comment, the report said.